In the manufacture of multiple-level printed circuit boards, either multilayer printed circuit boards or discrete wiring boards, it is necessary to insure registration of each layer with respect to all others.
As an example, in either multilayer printed circuit boards or discrete wiring boards, the "signal" layers, i.e., those containing the pathways for the logical interconnections, must be registered with each other, with the power-ground format layers and the input/output/mounting pad layers. The connections between these layers usually are made by drilling through the boards at the desired interconnection points and then metallizing the drilled holes in order to form the interconnections. If the layers are not in accurate alignment, the drilled holes will not pass through the intended connection points, and may, in fact, pass through terminal points or conductors which are not intended to be connected to these outer layers.
Registration is normally provided by the inclusion in each layer of two or more optically located "tooling" or registration holes which are intended to fit accurately over a pair of corresponding pins having the same diameter as the drilled "tooling" holes. These "tooling" holes and pins are repeated on the work stations of each critical manufacturing tool, i.e., the pattern printer (if multilayer printed circuit boards), the wiring machine (if discrete wiring boards), and the drilling machine.
In the production of sequentially laminated multilayer printed circuit panels, the achievement of front-to-back registration of individual inner layers is not normally a problem. The glass or metal-based photoprinting tools usually have extremely high edge definition and contrast. As a result, the first-side tool for one layer can be optically aligned with the second-side tool with excellent accuracy. Both sides of the thin inner layers are then photoprinted at the same time and, therefore, it is only necessary that care be taken in set-up of the two-sided printer to achieve good inner-layer registration.
The real difficulty, and the operation which is prone to produce scrap, comes in stacking the individual layers prior to lamination. Material shrinkage or stretch or warp gives rise to this difficulty; the panels are opaque or translucent, so that optical registration using features common to all layers (e.g. termination pads), is not usually feasible. Normally this operation is carried out by including a series of "targets" on each of the layers, formed at the same time that the photoprinter exposes (creates) the doublesided image on each inner layer surface. These "targets" are subsequently used to locate "tooling holes" for the purpose of locating, in conjunction with corresponding "tooling pins", the layers in the correct position relative to each other during lamination.
The finished inner layers are then stacked in proper order, together with the required ground, power and component mounting-pad layers, and with interleaved prepreg layers, on pins located on an assembly jig, in preparation for the lamination step. Unfortunately, during their manufacturing process, prior to laminating, the layers are subjected to heat during (i) conductor formation, (ii) hole metallization steps, and (iii) removal by etching of most of the copper foil which had restrained movement of the thin dielectric core. These factors introduce considerable dimensional variation into the layers, and therefore the original tooling targets are not at the same locations that they were when they were first formed.
Each of the laminating "tooling holes" may be individually and precisely located in reference to their respective target, but the layer's dimensional changes, resulting from the aforementioned manufacturing stresses, may cause the holes to be mislocated and fail to match the position of the "tooling pins". Forcing the layers onto the pins is a common cause for layer-to-layer misregistration.
In one optical alignment system, several "targets" (e.g. one in each corner of the layer) may be optically sighted and used to achieve a compromise position of the layer in a punching or drilling tool which generates all the "tooling holes" in the correct position to each other but in a "best compromise" position in relation to the distorted layer. While this method is generally considered an improvement, and has been incorporated in commercial embodiments (for example, in the Opti-Line PE equipment manufactured by the Multiline Division of Lenkeit Corporation), it is subject to the shortcomings of the optical systems previously described. In addition, the "best compromise" position is subject to human variability, if done by an operator, or to the sophistication of the optical system and image positioning algorithm in the case of computer controlled optical systems.
In either case physical and optical variations of the "targets" occur to some degree. These variations may be related to:
Size PA1 Reflectivity PA1 Contrast to background PA1 Form definition PA1 Within a target PA1 Among targets on the same layer PA1 On targets residing on different layers.
In addition, these variations may occur:
As a result of these variations, the "best compromise" perceived by either an operator or arrived at as a result of processing optically derived data does not necessarily coincide with the "best geometrical compromise".
Optical work positioning systems are suitable for many applications. In other applications, however, optical systems have been used because of the lack of available alternatives, but they present serious shortcomings. These shortcomings include (1) insufficient depth of field at the magnification needed to achieve the desired resolution, e.g., magnification of at least 100X is required for a resolution of about 0.0005 inch (0.0125 mm); (2) high cost due to the need for at least two expensive devices at each work station to define the geometrical position of features on one plane; (3) difficulty in mounting the optical devices on operating machines in a sufficiently rigid manner to retain the precise reference position intended in the presence of vibration and acceleration/deceleration; (4) the need for the operator to alternatively view two targets (on two optical devices), in order to effect the workpiece position, in an iterative manner, or, alternatively, the need of complex and expensive optical or television systems to accomplish the task non-iteratively; (5) the inability to access images (features) on the face resting on a machine table (down face) as necessary, for example, when a second side of an opaque panel has to be wired in registration with the first side; (6) the difficulty in averaging out or symmetrically splitting dimensional errors of the workpiece; and (7) the need for high optical contrast targets.
While the layer-to-layer registration errors attributable to these factors are generally acceptable for process tolerances of a few mils (tenths of a mm), they become intolerable in processing more advance, denser designs where the acceptable registration tolerances are in the vicinity of one mil (0.025 mm) and preferably lower.
Looking at the manufacturing process for a typical two-level wiring discrete wiring board, there are four critical operations where precision is required: (1) first side-to-side voltage plane/ground plane (format) registration; (2) side wiring to voltage/ground plane registration; (3) second side to first-side wiring registration; and (4) drilling to wire pattern registration.
The manufacture of the two-sided format comprising a voltage plane and a power plane involves conventional two-sided photo print image formation. Good two-side photo printers have been available for many years. The desired precision in this operation is readily achievable because of the transparency of the base film and the high optical contrast between this base and the photoimage formed thereon.
Wiring-to-format and wiring-side-one to wiring-side-two registration are characterized in that individual layers of features are generated in separate operations. Moreover, as each layer is produced the previous layer is not optically visible. For example, the step of registering the wiring to the format is "blind" because the underlying opaque format is covered by opaque or translucent adhesive. Similarly, when wiring the second layer, the first layer is face down on the table and cannot be used as a visual reference. The conventional way of performing this operation, i.e., using pins and matching holes in the panel, has certain flaws. These flaws include inaccuracies in pin/hole fit, material shrinkage or deformation and misadjustment in position of the pins with respect to the wiring head and/or x-y axis travel.
In spite of the considerable efforts that have been expended to insure registration of the layers throughout the manufacturing sequence, misregistration caused by errors in the tooling hole location or size, and/or material stretch or shrinkage remains a significant reason for the rejection of these multiple layer boards. As board size and interconnection density rises, this problem becomes even more serious.
Accordingly, it is the object of the present invention to provide a simple and accurate method of registering the various layers of either multilayer printed circuit boards or discrete wiring boards during their manufacture.
It is a further object of the present invention to provide a method of registering the various layers of either multilayer printed circuit boards or discrete wiring boards which is substantially free of tooling hole location or size defects and material stretch or shrinkage problems.
The present invention provides a solution to the shortcomings associated with the prior art mechanical or optical sensing systems described above.